--- a/thys/Recursion-Theory-I/Utils.ML
+++ b/thys/Recursion-Theory-I/Utils.ML
@@ -14,7 +14,7 @@
 fun extract_trueprop_arg (Const ("Trueprop", _) $ t)  = t
   | extract_trueprop_arg  _ = raise BadArgument
 
-fun extract_set_args (Const ("op :", _) $ t1 $ t2)  = (t1, t2)
+fun extract_set_args (Const (@{const_name Set.member}, _) $ t1 $ t2)  = (t1, t2)
   | extract_set_args  _ = raise BadArgument
 
 fun get_num_by_set "PRecFun.PrimRec1" = 1